Currying

object SpaceAge {
  private val EARTH_SECONDS = 31_557_600.0
  private def calculate(orbitPeriod: Double) =
    (seconds: Double) => seconds / EARTH_SECONDS / orbitPeriod

  val onEarth = calculate(1.0)
  val onMercury = calculate(0.2408467)
  val onVenus = calculate(0.61519726)
  val onMars = calculate(1.8808158)
  val onJupiter = calculate(11.862615)
  val onSaturn = calculate(29.447498)
  val onUranus = calculate(84.016846)
  val onNeptune = calculate(164.79132)
}

This approach starts by defining the number of earth seconds in a year. Note the use of digit separators (_) makes long numbers more readable.

The calculate() method is defined as a curried function. In this situation, the curried function is a function of two parameters defined as a function that takes the first parameter and returns another function which takes the second parameter.

The method could also be defined as a binding like so:

private val calculate = (orbitPeriod: Double) =>
  (seconds: Double) => seconds / EARTH_SECONDS / orbitPeriod

The slight change in syntax is exchanging the = operator for a => operator after the first parameter.

The method syntax could be read "The function named 'calculate' that takes the parameter 'orbitPeriod' has a body which is a function that takes the parameter 'seconds' and returns seconds / EARTH_SECONDS / orbitPeriod."

The function literal syntax could be read "The binding named 'calculate' is a function which takes the parameter 'orbitPeriod' and returns a function which takes the parameter 'seconds' and returns seconds / EARTH_SECONDS / orbitPeriod."

Following the calculate() method are bindings with the same names as the methods expected by the tests. The bindings are set to the calculate() method with the first argument passed for the orbit period. The calculate() method called with its first argument returns a function which takes the argument for seconds. So each binding is the calculate() method ready to accept its second argument.

When a test calls one of the bindings with the seconds value, it is calling the curried calculate() function that is expecting its second argument. The curried function takes the seconds value and returns the result of the expression in the body of the curried function.